Balance Rate description [Archive] - Diesel Place : Chevrolet and GMC Diesel Truck Forums

Terrain Twister

10-07-2004, 02:48 PM

Here ya go!

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Duramax Diesel 6.6L Diagnostic Tips (Changes)</TD>
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Here are some diagnostic tips you may find useful when diagnosing driveability concerns on the Duramax 6.6L diesel engine. This information replaces the July 2001 Techlink article on Duramax Diagnostic Tips.

Misfire
If there is a misfire code, or a misfire can be detected during operation, connect the Tech 2. Look on the Engine Data 2 list to find the Balancing Rates for the cylinders. Do not use Balancing Rates to diagnose symptoms that do not occur at idle. Use the fuel Injector Balance Test with Tech 2 to diagnose symptoms at higher engine and vehicle speeds.

Balancing Rates
Normal fuel delivery to each cylinder is around 1 cubic millimeter (mm3) for the pilot injection and 7 mm3 for the main injection. The balancing rates are the measurements of the amount of fuel removed from or added to these normal fuel quanti**** during each combustion event.

Balancing rates are available and accurate only during stable idle. These numbers tell you whether or not you have a possible compression, injector control, or injector issue.

Once the misfire is detected, the balancing rate of the cylinder that has the misfire will be as high as 15 mm3. The misfire codes (P0300, P0301-P0308) will set only after the first 90 seconds on Federal LB7s (30 seconds on California LB7) of engine operation and will set only when the balancing rate goes above 15 mm3.

The normal values that should be seen on the balancing rate parameters will range between -4.0 and +4.0 in Neutral or Park (-6.0 to +6.0 in Drive when the brake is fully applied). Suspect a possible problem when the balancing rate of a particular cylinder starts to increase out of the normal operating range. An example of a possible problem cylinder would be a balancing rate of 7 mm3 or so. Once the numbers start to increase out of the normal range, the cylinder is progressively contributing less power than the other cylinders.

Diagnosing "Fuel Knock"
Perform the diagnostics listed in the Fuel Knock symptom table.

Vacuum/Pressure Gauge
The vacuum/pressure gauge is also useful when diagnosing the Duramax 6.6L diesel (fig. 13). In the case of a complaint of “starts, runs, then dies,” or if the vehicle will not start after sitting for a period of 30 minutes or longer, hook up the vacuum/pressure gauge to the Schraeder valve on the right front valve cover to determine if air intrusion into the fuel system is the root cause of the complaint. With the vacuum/pressure gauge hooked up, prime the fuel manager until 10 psi is reached. If pressure does not reach 10 psi after twenty or so pumps, air intrusion is the cause. Refer to the fuel system diagnostic in the manual.

In the case of a “cranks, no start” condition, use the Tech 2 to check for codes. If no codes are present, use the vacuum/pressure gauge to check the fuel supply system vacuum to the pump while cranking. If the vacuum increase to more than 5 in. Hg, there is a restriction in the fuel system between the fuel tank and the high pressure pump. If the vacuum is ok, ensure that pressure is being made under Engine Data 1, looking at Actual Fuel pressure.

Observe the Engine Cranks but Does Not Run Diagnostic and follow the guidelines provided in order to determine if the pump or the fuel injectors are the problem.

- Thanks to William Smithers</TD></TR></